Indiana State Police(DELPHI, Ind.) — Authorities investigating the slayings of two girls in Indiana have received over 500 tips since making an impassioned plea on Wednesday for people to come forward with information, a law enforcement source close to the investigation told ABC News Thursday.

The surge in tips followed the release of a new video clip retrieved from the cellphone of one of the victims, 14-year-old Liberty "Libby" German. When police played some audio from the clip at a news conference in the town of Delphi on Wednesday morning, reporters heard just three words from a deep voice: "Down the hill."

The audio quality is not great, but police said it's enough for someone to recognize the individual's voice. Investigators believe the clip was recorded just before the attack.

"Libby had the presence of mind to turn on her video camera," Indiana State Police spokesman Capt. David Bursten said Wednesday. "There's no doubt in our minds that that young lady is a hero."

The rest of the video will not be released at this time because of the ongoing investigation. Investigators recovered other evidence from the girl's phone that is also not being released, Bursten said.

German and 13-year-old Abigail "Abby" Williams, both of Carroll County, were reported missing by their families Feb. 13 after the two did not return from a hike.

After organized searches, the bodies of the two girls were found Feb. 14 outside Delphi in the woods near Deer Creek, about three-quarters of a mile from an abandoned railroad bridge where they were dropped off the day before to go hiking. An autopsy revealed their identities.

"Evidence in this case has led investigators to believe that this is a double homicide, and that's what we're investigating at this time," Indiana State Police spokesman Sgt. Tony Slocum told reporters Wednesday.

Indiana State Police on Monday said a man in a photograph is the primary suspect in the investigation. The man — dressed in blue jeans, a blue jacket and a hoodie — was photographed on a nature trial around the same time the two girls disappeared.

He was previously labeled a person of interest, and police had said he might be only a witness to the crime.

"We are actively looking for this person. We believe this person is our suspect," Slocum said.

Authorities spent much of Wednesday's news conference delivering an emotional appeal to the public to submit tips to track down the man in the photograph and the person whose voice is in the audio clip.

Investigators said that there is the possibility of more than one suspect and that it's unclear whether the voice in the audio clip belongs to the man in the photograph.

The FBI has been assisting local authorities in the investigation since last week. Agents have briefed FBI Director James Comey on the case on two occasions.

Gregory Massa, the FBI assistant special agent in charge in Indianapolis, asked the public to think back to Feb. 13, the day the girls went missing.

"Just think if you had an interaction with an individual who inexplicably canceled an appointment that you had together," Massa said Wednesday. "Or an individual called into work sick and canceled a social engagement. At the time, they gave what would have been a plausible explanation."

Suspicious behavior or a change in someone's behavior should also be a red flag, Massa said.

"Did [an] individual travel unexpectedly?" he asked. "Did they change their appearance? Did they shave their beard, cut their hair or change the color of their hair? Did they change the way they dress?"

Authorities and community leaders are offering a $41,000 reward for any information leading to an arrest.

Citizens can provide information about this case by calling the Delphi murder tip line at 844-459-5786. Information can be reported anonymously. Tips can also be emailed to abbyandlibbytip@cacoshrf.com.

Check Also

ABCNews.com(LOS ANGELES) -- At least 1,000 California residents were forced to evacuate on Monday evening due to a 2,000-acre wildfire that had already destroyed one structure, officials said.California fire officials issued a mandatory evacuation for ...